The present invention relates to a sealed structure having a structure of sealing an element on a silicon board by a glass plate, and a method of fabricating a sealed structure, and a semiconductor device, and a method of fabricating a semiconductor device.
Although there are present various kinds of elements formed on a board, or mounted on a board, depending on the kinds of the elements, there is a case in which it is preferable that the elements are used in a state of being sealed on the board.
For example, an element using a microelectromechanical system (there is a case of being referred to as ‘MEMS’) (referred to as ‘MEMS element’ as follows) is a representative element which is preferably used by being sealed on a board in view of a structure thereof.
As such an MEMS element, there is an optical functioning element of, for example, a pressure sensor, an acceleration sensor, a digital micromirror device or the like. It is preferable to use such an MEMS element in a vacuum state, a reduced pressure state, or an atmosphere replaced by an inert gas, it is preferable to use the element by being sealed in airtight. Therefore, a method of sealing an MEMS element is proposed (refer to, for example, Patent Reference 1).
According to a semiconductor device described in Patent Reference 1, there is used a sealed structure constituted by bonding a board and a support plate by anodic bonding in a state of mounting a support plate (glass) having a recessed portion on a silicon board mounted with an element and covering the element by the recessed portion.
[Patent Reference 1] JP-A-2005-166909
However, according to the semiconductor device of the background art, in the anodic bonding, by applying a high voltage in a state of bringing an anode into contact with a lower face (back face) of the silicon board and bringing a cathode into contact with an upper face of the support plate, there poses a problem that in a procedure of softening glass including movable ions by heating the silicon board and simultaneously bonding the silicon board and the glass by an electrostatic attraction force, an impurity (sodium (Na+)) included in glass is precipitated on a side of the cathode of a glass face.
When an opaque layer by sodium is formed on the glass face in this way, for example, in a case of containing an element for receiving or emitting light such as an optical functioning element in a sealed space, there is a concern of reducing transmittance of light. Further, sodium is difficult to be removed from the glass surface since sodium is provided with an adherence, and therefore, it is difficult to recover the transmittance of glass even when the glass is cleaned or the like.
Hence, in view of the above-described situation, it is an object of the invention to provide a sealed structure, and a method of fabricating a sealed structure, and a semiconductor device, and a method of fabricating a semiconductor device resolving the problem by preventing an impurity from being precipitated on a glass surface.